Electrical circuit comprising an electromagnetic relay and a switching arrangement which is mounted in parallel to a contact of the magnetic relay

ABSTRACT

The invention relates to an electrical circuit ( 1 ) comprising an electromagnetic relay ( 4 ) and a switching arrangement ( 5 ), the outputs (A 1 , A 2 ) of said switching arrangement being mounted in parallel to a contact ( 4   a ) of the electromagnetic relay ( 4 ). A control system ( 2 ) is connected to the coil ( 4   b ) of the relay ( 4 ) and the switching arrangement ( 5 ). The aim of the invention is to create one such circuit in a relatively interference-free manner. To this end, a voltage detection unit ( 6 ) is arranged between the control system ( 2 ) and the coil ( 4   b ). A first switching signal (S 1 ) is produced, at a switching-on command, by means of a control unit ( 7 ) which is arranged downstream from the voltage detection device ( 6 ), said signal short-circuiting the outputs (A 1 , A 2 ) of the switching arrangement ( 5 ). At the end of the switching-on command, the outputs (A 1 , A 2 ) of the switching arrangement ( 5 ) remain short-circuited until the contact ( 4   a ) of the relay ( 4 ) is opened. In the absence of a switching-on command, a second switching signal (S 2 ) is produced by means of the voltage detection device ( 6 ) and the control unit ( 7 ), eliminating the short-circuiting of the outputs (A 1 , A 2 ) of the switching arrangement ( 5 ).

CLAIM FOR PRIORITY

This application is a national stage of International Application No.PCT/DE03/00070 which was published on Jul. 31, 2003 and which claims thebenefit of priority to German Application No. 102 03 682.9 filed Jan.24, 2002.

TECHNICAL FIELD OF THE INVENTION

The invention relates to an electrical switching arrangement having anelectromagnetic relay, a switching device, whose outputs are arrangedparallel to one contact of the electromagnetic relay, and a controlarrangement which is connected to the coil of the electromagnetic relayand to the switching device.

BACKGROUND OF THE INVENTION

Such an arrangement is disclosed as a so-called instantaneous releaseunit in the appliance manual “Schutztechnik—Digitaler Oberleitungsschutz7SA518/519 V3.2” [Protective relaying—digital overhead line protection7SA518/519 V3.2] issued by Siemens AG, November 1999 (cf. pp. 3-6, R-13and R-15). This instantaneous release unit serves the purpose ofreducing the switch-on time of a downstream protective device. For thispurpose, the outputs of a switching arrangement are arranged parallel toone contact of an electromagnetic command relay. Both the switchingarrangement and the command relay are connected to a control unit viaisolated connecting lines. If the control arrangement emits a switchingcommand, both the contact of the electromagnetic relay and the output ofthe switching arrangement are short-circuited. The switching arrangementis designed such that its switch-on time is markedly shorter than thatof the electromagnetic relay, with the result that the switch-on time ofthe switching arrangement and not that of the electromagnetic relaydefines the switch-on time of the downstream protective device.

SUMMARY OF THE INVENTION

The invention improves upon the above-described switching arrangement interms of its lack of sensitivity with respect to electromagneticinterference and, on the other hand, markedly reducing the load on therelay contact owing to arc flashovers.

In one embodiment of the invention, there is a voltage detection devicebeing arranged between the control arrangement and the coil of theelectromagnetic relay, the voltage detection device instructing, in theevent of a switch-on command being emitted by the control arrangement, adownstream drive unit to emit a switching signal which short-circuitsthe switching device on the output side, maintaining, when the switch-oncommand is ended, the switching signal until the contact of theelectromagnetic relay is opened, and instructing, in the event of therebeing no switch-on command, the drive unit to emit a second switchingsignal which opens the switching device on the output side. By thismeans it is possible both to ensure a rapid switch-on time of downstreamdevices and to protect the contact of the relay by the electrical powerbeing drawn completely by the switching device when the relay contact isopened. In addition, the arrangement according to the invention reliablyprevents the outputs of the switching arrangement from beinginadvertently short-circuited owing to external interference and thusprevents downstream devices from being driven in an undesired mannerwhen there is no switch-on command for the relay.

One advantageous emboidment of the arrangement according to theinvention is the voltage detection device having a rectifier circuitwhich is connected on the input side to the control arrangement and thecoil of the electromagnetic relay and is connected on the output side tothe drive unit via a comparator. In this case, both in the event of aswitch-on command from the control arrangement being present and in theevent of a voltage being produced by induction in the coil of theelectromagnetic relay during the switch-off procedure, the downstreamcomparator receives, via the rectifier circuit, a clear signal whichinstructs the comparator to drive the downstream drive unit such that itshort-circuits the switching device at the beginning of a switch-oncommand and maintains the short-circuited state of the switching devicefor a specific period of time when the switch-on command is ended.

A voltage is advantageously continuously applied to one input of thecomparator. When selecting a voltage such that it has the oppositepolarity to the voltage emitted to the comparator by the rectifiercircuit, it is possible to achieve the situation in which the comparatoremits a clear signal to the downstream drive unit which instructs saiddrive unit to open the switching device.

In one advantageous embodiment of the arrangement according to theinvention, the drive unit has two signal conversion elements driven inphase opposition in such a way that in each case one signal conversionelement is active and one signal conversion element is inactive.

This means that it is easily possible, in the case of a first signalemitted by the comparator, for the drive unit to be instructed to closethe switching device, and, in the case of a second signal emitted by thecomparator having the opposite mathematical sign to the first signal,for the drive unit to be instructed to open the switching device.Depending on the mathematical sign of the signal emitted by thecomparator, either one or the other signal conversion element is drivenin this embodiment.

A further advantageous embodiment of the arrangement according to theinvention provides for the outputs of the respectively inactive signalconversion element to be short-circuited via the respectively activesignal conversion element. The respective short-circuiting of the signalconversion element which at that time is not emitting a signal furtherreduces the susceptibility of the arrangement to electromagneticinterference.

The signal conversion elements may advantageously be in the form ofvoltage transformers.

Alternatively, the signal conversion elements may advantageously be inthe form of photovoltaic generators.

The switching device advantageously has at least one MOS transistor.This makes it possible to provide the required switching power at a lowdrive power after a relatively short switch-on time.

A further advantageous embodiment of the switching arrangement accordingto the invention provides for the switching device to operatebi-directionally. This makes it possible for both direct currents andalternating currents to be connected to the output of the switchingarrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with reference to thedrawing which illustrates one exemplary embodiment of the arrangementaccording to the invention, in which:

FIG. 1 shows a block circuit diagram according to an embodiment ofinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a switching arrangement 1 has a control arrangement 2which is used to switch on a device (not illustrated in the figure)which is connected to outputs 3 of the switching arrangement 1. Theoutputs 3 of the switching arrangement 1 are arranged parallel to onecontact 4 a of an electromagnetic relay 4 and to the output of aswitching device 5. The coil 4 b of the electromagnetic relay 4 isconnected directly to the control arrangement 2. A voltage detectiondevice 6, whose output is connected to the switching device 5 via adrive unit 7, is located between the control arrangement 2 and theelectromagnetic relay 4.

The arrangement functions as follows: If a switch-on command is emittedby the control arrangement 2, the coil 4 b of the electromagnetic relay4 is excited and closes the contact 4 a. Owing to the mechanical inertiaof the contact 4 a, the electromagnetic relay 4 has a switch-on timewhich is usually greater than 5 ms. At the same time as the coil 4 b ofthe electromagnetic relay 4, the voltage detection device 6 receives theswitch-on command from the control arrangement 2. The voltage detectiondevice 6 passes an actuating signal B on to the drive unit 7 in which afirst switching signal S1 is generated, as a result of which the outputsA1 and A2 of the switching device 5 are short-circuited. Since theswitching device 5 generally comprises electronic components, it has amarkedly shorter switch-on time than the electromagnetic relay 4. Whenthe switch-on command is emitted by the control arrangement 2, first ofall the switching device 5 thus short-circuits its outputs A1 and, A2and draws the full power for driving the device connected to the outputs3. As soon as the contact 4 a of the relay 4 is closed, a significantproportion of the power may be passed through the closed contact 4 a.

When the switch-on command emitted by the control arrangement 2 isended, the magnetic field in the coil 4 b of the electromagnetic relay 4is dissipated. As a result, a back-e.m.f. is induced at the inputs ofthe coil 4 b and is detected by the voltage detection device 6. Theinduced voltage is applied until the magnetic field in the coil 11 hascompletely dissipated and the contact 4 a of the electromagnetic relay 4is thus opened. As long as the induced voltage is applied, by means ofthe voltage detection device 6 the actuating signal B at the drive unit7 and thus the switching signal S1 for short-circuiting the outputs A1and A2 of the switching device 5 are maintained. As the contact opens,an arc is prevented from forming at the contact 4 a of theelectromagnetic relay 4 by the fact that the current can continue toflow via the closed output of the switching device 5. When the magneticfield has dissipated and the contact 4 a of the electromagnetic relay 4is completely open does the switching device 5 also reverse the shortcircuit at its outputs A1 and A2.

If the control arrangement 2 does not emit or maintain a switch-oncommand, a further switching signal S2 is emitted by the voltagedetection device 6 via the drive unit 7 to the switching device 5 forthe purpose of opening the outputs A1 and A2 of the switching device 5or for the purpose of keeping them open. This prevents the switchingdevice 5 from being inadvertently switched on owing to electromagneticinterference.

The ways in which the voltage detection device 6, the drive unit 7 andthe switching device 5 function will be explained in more detail belowwith reference to the figure. The voltage detection device 6 includes arectifier circuit 13 and a comparator 14. Both in the event of aswitch-on command and in the event of an induced voltage being present,the rectifier circuit 13 provides a positive voltage across thecomparator module 14 which thereupon likewise emits a so-called “high”signal, i.e. a positive back-e.m.f. at its output A3. If neither aswitch-on command nor an induced voltage are present, a negative voltagetakes effect at a further terminal 15 of the comparator 14; thecomparator 14 thus emits a negative voltage at its output A3. Thenegative voltage at the terminal 15 is in this case of such a value thatit is smaller than the positive voltage emitted by the rectifier,circuit in the event of a switch-on command from the control arrangement2.

Two signal conversion elements 16 and 17, which are connected preciselyin phase opposition with the aid of an inverter 18 to the output A3 ofthe comparator 14, are located in the drive unit 7. In the figure, thesignal conversion elements 16 and 17 are illustrated as photovoltaicgenerators. If the comparator 14 provides a high signal, a voltage isgenerated at the output of the signal conversion element 16. By means ofthe inverter 18, the signal conversion element 17 receives a “low”signal, i.e. no voltage at its input, and thus does not produce anoutput voltage. If the comparator 14 emits a low signal at its output,the behavior of the signal conversion elements 16 and 17 is preciselyreversed: The signal conversion element 16 does not emit an outputvoltage, whereas a voltage is applied to the output of the signalconversion element 17. The drive unit 7 also contains transistors 20 and21, by means of which in each case the outputs of the signal conversionelement (16, 17) which does not emit a voltage at its output can beshort-circuited. This further reduces the susceptibility toelectromagnetic interference.

The switching device 5 may (as illustrated in the figure) have two powerMOS transistors 22 and 23 which are connected to one another at theirsource terminals, whereas the two drain terminals of the powertransistors 22 and 23 form the outputs A1 and A2 of the switching device5. The gate terminals of the power transistors 22 and 23 are connectedto one another and to the positive output of the signal conversionelement 16, whereas the two source terminals are connected to thepositive output of the signal conversion element 17. If the signalconversion element 16 now produces a voltage at its output (this is thecase when a high signal is emitted by the comparator 14), a positivevoltage is applied at the gate terminals of the power transistors 22 and23 and the transistors 22 and 23 are turned on. A current can thus flowvia the outputs 3 of the switching arrangement 1. If the signalconversion element 17 produces an output voltage (this is the case whena low signal is emitted by the comparator 14), a positive voltage withrespect to the gate terminal is applied to the source terminals of thepower transistors 22 and 23, and the power transistors block the currentflow.

As an alternative to this, it is also possible for the power transistors22, 23 to be turned off by the source/gate terminals of the powertransistors 22, 23 being short-circuited. For this purpose, theconnection between the positive output of the signal conversion element17 and the source terminals of the power transistors 22, 23 between thepoints P1 and P2 needs to be removed and, in its place, the sourceterminals of the power transistors 22, 23 at point P2 need to beconnected to the source terminal of the transistor 20 at point P3.

1. An electrical switching arrangement comprising: an electromagneticrelay; a switching device, having outputs arranged parallel to onecontact of the electromagnetic relay; a control arrangement which isconnected to a coil of the electromagnetic relay and the switchingdevice; and a voltage detection device is arranged between the controlarrangement and the coil of the electromagnetic relay, the voltagedetection device instructing, when a switch-on command is emitted by thecontrol arrangement, a downstream drive unit to emit a switching signalwhich short-circuits the switching device on the output side,maintaining, when the switch-on command is ended, the switching signaluntil a contact of the electromagnetic relay is opened, and instructing,when there is no switch-on command, the drive unit to emit a secondswitching signal which opens the switching device on the output side. 2.The electrical switching arrangement as claimed in claim 1, wherein thevoltage detection device has a rectifier circuit which is connected onthe input side to the control arrangement and the coil of theelectromagnetic relay and is connected on the output side to the driveunit via a comparator.
 3. The electrical switching arrangement asclaimed in claim 2, wherein a voltage is continuously applied to oneinput of the comparator.
 4. The electrical switching arrangement asclaimed in claim 1, wherein the drive unit has two signal conversionelements driven in phase opposition such that in each case one signalconversion element is active and one signal conversion element isinactive.
 5. The electrical switching arrangement as claimed in claim 4,wherein the outputs of the respectively inactive signal conversionelement are short-circuited via the respectively active signalconversion element.
 6. The electrical switching arrangement as claimedin claim 4, wherein the signal conversion elements are voltagetransformers.
 7. The electrical switching arrangement as claimed inclaim 4, wherein the signal conversion elements are photovoltaicgenerators.
 8. The electrical switching arrangement as claimed in claim1, wherein the switching device has at least one MOS transistor.
 9. Theelectrical switching arrangement as claimed in claim 1, wherein theswitching device operates bi-directionally.